1. Field of the Invention
The present invention generally relates to a leak diagnostic apparatus for an evaporative emission control system that purges fuel vapor (evaporated fuel) from a fuel tank to an intake passage of an engine.
2. Background Information
In order to prevent fuel vapor from being discharged to the atmosphere, a known evaporative emission control system directs fuel vapor from inside a fuel tank through a fuel vapor vent passage to a canister where the fuel is adsorbed. The fuel evaporative emission control system then purges the adsorbed fuel vapor to an intake passage of an engine. In this kind of evaporative emission control system, the amount of fuel vapor that is purged to the intake passage is adjusted by controlling the opening degree of a purge valve provided in a passage communicating between the canister and the intake passage.
A known method of diagnosing such an evaporative emission control system for leakage is to close the purge valve such that the space from the fuel tank to the purge valve is sealed and determine if a leak exists based on a pressure change occurring inside the sealed space. However, there are times when the fuel tank changes shape due to a difference between the internal and external pressures of the fuel tank, thus causing the volume of the fuel tank to change. The change in volume can affect the pressure in the sealed space and cause an incorrect diagnosis to occur. Therefore, the technology disclosed in Japanese Laid-Open Patent Publication No. 2003-83176 is contrived to detect a pressure inside the fuel tank during a leak diagnosis and stop the leak diagnosis if a pressure change indicative of a large change in the shape of the fuel tank occurs.